The invention relates to cutting tools used on mining and construction machines to break hard natural materials such as rock and minerals (e.g., coal) and also man-made materials such as concrete and asphalt.
Known in the industry is a prior art cutting tool of that type which comprises a hard alloy cap having a base mounted on a metal shank (e.g., see U.S. Pat. No. 4,938,538). The base of the hard alloy cap is shaped in such a way as to provide the tool with sufficient strength and durability for its intended operating conditions. Also known in the industry is the fact that caps have a better geometric shape than inserts, which improves the cutting efficiency of the tool by reducing the cutting forces needed to break the material. Further known is the fact that caps increase the operating life of prior art tools by better protecting the metal shank from the material being cut.
However, the geometric form of the cap is not optimal for the operating life of the tool. A prior art tool depicted in FIGS. 1 and 2 comprises a steel tool body 10 and a cap 11 of hard metal. The cap 11 has a conical tip 12, and a base 13 intended to rest against a supporting surface 14 on the tool body 10, to protect the portion of the steel tool body 10 surrounding the cutting cap 11 from such wear as would cause the cap 11 to become loose. The rear contact surface 20 of the base 13 is brazed to the supporting surface 14. The cap 11 is provided with an intermediate portion 15 located between the tip 12 and the base 13.
The intermediate portion 15 comprises a cylindrical intermediate surface portion 17", and a concave portion 17'. Due to the elongated intermediate surface portion 17" the required cutting force is maintained low even when the tip portion 12 becomes worn since the tip size remains generally the same as the tip wears down along the elongated intermediate surface portion 17". Due to this design it is also ensured that the steel in the tool body 10 surrounding the cutting insert is protected against premature abrasion; this protection being provided by the concave portion 17' and the base 13. The base 13 has a diameter D, and the intermediate surface portion 17" has a diameter d. A distance H extends from the front of the tip portion 12 to a rear end of the base 13. A ratio of H/D is less than 1.0, and a ratio of d/D is less than 0.7.
Despite the successful performance of that cap 11, room for improvement remains. Often times the cap wears down to a shape that increases the cutting forces so much that the tool becomes unusable. Larger caps can increase the life of the tool, but since the hard alloy material is usually an expensive tungsten-cobalt material, the cost of the tool also increases.
An object of the invention is to provide a cap geometry which reduces the cost of the tool by using less tungsten-cobalt material while increasing the life of the tool by maintaining lower cutting forces longer, and at the same time protecting the metal shank from the material being cut.